Our long term goal is to elucidate the role of 9-O-acetylated sialic acids in T cells, both in normal physiology and in the setting of immune-mediated diseases and malignancy. Several studies suggest specific roles for 9-0-acetylated sialic acids in tissue development and organization, as well as specific enzymatic mechanisms for their generation. 9-O-acetylation can affect biologic properties of underlying molecules, altering host-pathogen recognition, modulating activation of complement and masking ligands for lectins that mediate cell adhesion. Recent studies reveal that a subset of murine CD4 cells are preferentially 9-O- acetylated, and suggest a discreet role for 9-O-acetylated sialic acids in T cell ontogeny and function. We aim specifically to elucidate the expression, regulation and function of 9-O-acetylated sialic acids on T cells and their subsets. Such insights will enhance our understanding of both normal and pathologic T cell activity, and will likely have broader implications for research in T cell-mediated diseases and cancer. We will define the expression of 9-O-acetylated sialic acids on T cells and their subsets in human and other species to determine whether this modification is conserved during mammalian evolution. We will also examine T cells of patients with malignancy and patients with HIV infection to establish whether 9-O-acetylation is altered in these diseases. Regulation of T cell 9-O-acetylation will be analyzed using three murine models, each genetically engineered to lack expression of a single sialyltransferase. These mice are deficient in the sialylated cell-surface trisaccharides produced by each enzyme, and exhibit aspects of immunodeficiency and aberrant T cell survival. For these studies, we will use a recombinant soluble form of the Influenza C virus hemagglutinin-esterase as a probe for 9-O- acetylated sialic acids in both immunohistologic and flow cytometric assays. We will also investigate the function of 9-O- acetylated sialic acids by analyzing selectin binding in vitro and tissue homing patterns (tracking cells stained with lipophilic dye in vivo) of murine T cells that have been enzymatically de-O- acetylated. Finally, employing a construct that includes the CD4 gene silencer, we will generate a transgenic mouse with overexpression of a viral sialic acid specific 9-O-acetylesterase. These mice should selectively lack 9-O-acetylation on CD4 positive cells, and provide insights into the physiologic roles of 9-O- acetylated sialic acids in T cells.